Methods of extracting hydrogen from a gas

ABSTRACT

The method for extracting hydrogen from a gas stream containing hydrogen with or without other gases by a sorption-desorption process using as the hydrogen sorber a pellet bed of a porous, metallurgically bonded, heat-ballasted hydridable mixture comprising a major proportion of a heat-ballast material and a minor proportion of a hydridable metallic material.

This is a division of application Ser. No. 279,713 filed July 2, 1981,now U.S. Pat. No. 4,589,919.

The present invention is directed to a heat ballasted hydridablematerial and to the use of the material in conductinghydride/dehydriding cycles under improved kinetics.

BACKGROUND OF THE INVENTION AND PRIOR ART

In recent years considerable research effort has been expended ininvestigating the properties of hydridable materials, for example,materials having the structural formula of LaNi₅. These fascinatingmetallic materials have the capability of absorbing large amounts ofhydrogen and of releasing their hydrogen content, i.e., they arereversibly hydridable. The use of such materials as hydrogen-storagemedia has been visualized as being industrially important, particularlyin view of the fact that the materials can absorb hydrogen to a densitygreater than that which is provided by liquid hydrogen. It has beenfound that a number of engineering problems are encountered in dealingwith these materials which must be solved before satisfactory commercialdevices capable of repeatedly absorbing and desorbing hydrogen foruseful purposes can be devised. It is found that when these materialsare subjected to hydrogen and hydrogen is absorbed that heat isgenerated i.e., the reaction is exothermic. However, in order topersuade the resulting metal hydride to release its hydrogen content,the hydride must be heated, i.e., dehydriding is endothermic. It hadbeen believed that an industrial size device designed to handlesubstantial quantities of hydrogen would require transport ofconsiderable quantities of heat in or out of the device depending uponwhether the device was in an hydrogen-absorbing or hydrogen-releasingmode. The apparent necessity for the provision of elaborate heattransfer means in order for the device to operate would have meant acomplex and expensive device with many tubes, valves and pumps.

Another factor requiring consideration was based on the observationthat, during repeated hydriding and dehydriding cycles, the hydridablematerials (which may initially be relatively large in particle size)crack and undergo decrepitation due to the change in volume whichaccompanies the hydriding/dehydriding cycles. The finely divided debrisresulting from the decrepitation reaction provides additional problemsin containment and complicates the design of the containment device interms of valves, filters, etc. In addition it has been found that thefine decrepitated powder resulting from the action of thehydriding/dehydriding cycle upon the hydridable material tends to packin the containment device with high impedance to gas flow, andundesirable increases in pressure within the device unless appropriatedesign steps are taken to compensate for this effect. It has been foundthat the problem of moving heat in and out of a device containing thehydridable material can be minimized by including a heat storage mediumwith a hydride former, and this forms the subject matter of U.S. patentapplication Ser. No. 011,194 filed Feb. 12, 1979 now U.S. Pat. No.4,566,281. However, the problem of decrepitation of the hydride formerleading to excessive bed packing, blockage of filters, difficulty withvalves, etc. still remains. In addition, the nature of the heat storagemedium to be employed in connection with the hydride former is stillsusceptible of improvement. It is known from U.S. Pat. No. 4,110,425that various hydrogen-storage materials of types such as magnesium,titanium, vanadium and niobium and alloys such as those of lanthanum andtitanium with cobalt, nickel and iron may be bonded with variousplastics and remain useful as hydrogen-storage media. It would beexpected, however, that plastics would have limited strength and limitedcapability to resist elevation in temperature. Plastic-bonded pelletsplaced in beds of any substantial size and weight could thus be expectedto slump or creep with accompanying reduction in bed permeability to gasflow. Furthermore, the potential for gas evolution from the plasticbinder over an extended period of time exists and such gas evolutioncould serve as a contaminant in applications wherein hydrogen of highpurity is required. The afore-mentioned patent refers in turn to U.S.Pat. Nos. 3,669,745 and 3,881,960 which are directed to electrodes foruse in galvanic cells comprising mixtures of a hydride former and ofanother metal in powder form which are pressed and sintered U.S. Pat.No. 4,036,944 is also directed to hydrogen sorbent compositionscomprising lanthanum-nickel bonded with a plastic composition. Thispatent also contemplates the inclusion of minor amounts of coppet,nickel and iron metals in the plastic bonded compact. The patent reportson a failed experiment wherein 50 wt percent of copper powder wasintroduced with the LaNi₅. After two cycles of hydrogenation theresulting compacts disintegrated.

SUMMARY OF THE INVENTION

The invention is directed to powder metallurgical mixtures of a hydrideforming metallic material in a minor proportion with a major proportionof a metal which is essentially non-hydridable and acts as a heatballast. The powder metallurgy compacts produced in accordance with theinvention are individually porous and beds made of the individualcompacts or pellets are porous on a macroscale. The material provided inaccordance with the invention may be employed, as an example, inconducting cyclic hydride/dehydride reactions with improved kinetics.The porous compacts provided in accordance with the invention areessentially resistant to decrepitation over long periods of time, andbeds made of the compacts are readily permeable to gas passage.

DETAILED DESCRIPTION OF THE INVENTION

A hydridable material employed in accordance with the invention mayinclude pure metals, such as magnesium, titanium, vanadium, niobium andpalladium and binary, tertiary and more complex alloys of, for example,rare earths, titanium, cobalt, nickel, iron, zirconium, magnesium,calcium, manganese and mixtures and other combinations thereof.Illustrative examples are LaNi₅, LaNi₄.7 Al₀.3, LaNi_(5-x) Al_(x),LaCo₅, LaNi₃ Co₂, FeTi, (Fe_(1-x), Mn_(x)) Ti, Mg₂ Ni and Ti₂ Ni.Metallic materials in powder from which may be mixed with theaforementioned hydridable material as ballast include in particulariron, nickel, copper and aluminum. Alloys or mixtures of these metalsmay also be employed in powder form. The heat-ballast metal also servesthe function of binder to hold the composite, porous, powdermetallurgical structure together. Care should be taken to avoid thepotential for exothermic reaction between the metal binder or ballastand the hydridable material. For example, a reaction between aluminumand LaNi₅ may initiate at temperatures as low as 300° C. Preferably, theheat-ballast metal comprises at least about 60%, by volume, of thepellets.

In preparing compacts in accordance with the invention, a powderedhydridable material having a particle size of about 1 to about 700microns, e.g., about 10 to about 300 microns, is mixed with the ballastmetal or alloy powder also having a particle size in the same range; ispressed, for example, at pressures in the range of up to about 20,000lbs/in² or higher and is sintered, if necessary, in the range of about400° to about 1100° C., appropriate to the metallurgical nature of thecompact ingredients. The resulting compacts may be crushed to providepellets of an average size in a range of about 1 to about 10millimeters. Crushing of larger compacts to provide such pellets alsoprovides fresh, fractured faces and overcomes metal smearing effects ofthe powder mix against the die which may occur with softer metals suchas aluminum and copper. Alternatively, pellets may be formed directly bypowder metallurgical methods which have sizes in the range of ahout 2 toabout 30 millimeters. Tabletting, briquetting, roll compaction, etc. maybe employed. Porosity of individual pellets will usually range in thevicinity of about 20% to about 60% pores or voids. Preferably theporosity falls in the range of about 20% to about 40%, by volume. Suchpellets are readily permeable to hydrogen. In the physical structure ofthe pellet itself, the hydridable particles are readily available forcontact with gas and despite the fact that there is a smaller amount ofhydridable material than of ballast material in the pellet the hydridingand dehydriding reactions proceed rapidly. Since the ballast material isin contact with the hydridable material, flow of heat from thehydridable material to the ballast material or in reverse directioninvolves a very short distance indeed, leading to improved kinetics.

In carrying out a hydriding-dehydriding process, as for example, therecovery of substantially pure hydrogen from a hydrogen-containing gasstream also containing undesirable impurity gases, thehydrogen-containing stream will usually be under pressure. Thehydride-dehydride device containing a bed of heat-ballasted hydridablematerial provided in accordance with the invention will have atheoretical maximum capacity for hydrogen, an ascertainable total heatof reaction for capacity storage and an ascertainable heat capacity. Thebed will possess an equilibrium absorbing pressure which rises withtemperature. Sufficient heat storage capacity should be available in thebed in relation to the quantity of hydridable material that theequilibrium absorbing pressure will not reach the supply pressure ofhydrogen until 60% or more of the hydrogen storage capacity of thesorbent material is realized. The heat storage capacity of the bedadvantageously will be sufficient to permit absorption of thetheoretical maximum heat of the hydrogen sorbing reaction without theequilibrium absorption pressure exceeding the supply pressure of thehydrogen gas. Once the bed is hydrided at the supply pressure of thehydrogen gas, hydrogen of substantial purity may be recovered from thebed utilizing the reaction heat stored therein. Efficiency of hydrogenextraction from the bed will be inversely proportional to the rate ofhydrogen extraction. No auxiliary heat transfer means is required,although auxiliary heat transfer means or insulation may be employed.

It is found in preparing pellets in accordance with the invention thatthe oxygen content of the ballast metal powder should be carefullycontrolled. Thus, the oxygen content of the ballast metal powder shouldnot exceed about 0.1 percent by weight. More highly oxidized powders canbe improvHbvHd in relation to oxygen content by reduction in hydrogen attemperatures up to about 500° C. Such a reducing step may involve somesintering of the powder and necessitate crushing to provide metal powderof the appropriate particle size. Of course in such a situation careneeds to be taken to avoid oxidation of the metal powder during crushingor other processing to provide composite pellets. Such reoxidation canbe corrected by moderate heating of the composite particles in areducing atmosphere low in hydrogen.

It will be appreciated that aluminum oxide is not reducible withhydrogen. Other ballast metals and/or alloys possess differentadvantages including the potential for absorbing or catalyticallyconverting certain gaseous impurities in the hydrogen feed stream whichcould have the potential for poisoning the hydridable material. Thepowder metallurgy compacts of the invention may also contain up to about30%, by volume, of inert, refractory powdered material such as alumina,sillica, magnesia, etc. The inert material should have a particle sizein the range of about 50 to about 500 microns and sufficient metalbinder should be present to provide a strong, metal-bonded pellet.

Oxygen has been found to play a destructive role in relation toreduction of the hydrogen capacity of hydridable materials employed incomposites of the invention. Such hydridable materials are readilyoxidizable, and oxidized material is no longer hydridable. It appearsthat hydrogen-reducible oxygen associated with the metal ballast isreduced when the bed is hydrided. Hydriding is accompanied by a rise intemperature and the water generated during reduction of the metal oxideapparently reacts with the hydridable material with accompanying loss ofcapacity.

EXAMPLE I

A number of pellets were produced in accordance with the invention usingas ballast metals varying amounts of powdered iron, copper, aluminum andnickel. In each case LaNi₅ was employed as the hydridable material. Themetal powders had fine particle sizes of less than 44 microns and theLaNi₅ was reduced to a fine particle size by severalhydriding-dehydriding cycles. The powders were mixed by wire blendingand the mixes were pressed at 20,000 psi isostatically to provide 10 kgbillets of various sizes up to 10 kg. Pressing and sintering parametersare provided in Table I. The billets were crushed pior to use to pelletshaving a particle size range of about -4, +20 mesh.

The capacity data in Table I which are tabulated in the column ΔH/Mwherein H is equal to the gram atoms of hydrogen in the hydridablematerial and M is equal to the gram atoms of hydridable material wereobtained in closed or a deadend apparatus over at least fivehydriding-dehydriding cycles under quasi-isothermal conditions. It is tobe seen from the data that the highest ΔH/M value is that obtained onthe aluminum bonded material, thereby indicating that, under the ΔH/Mcriterion, aluminum is the most satisfactory bonding metal to beemployed in accordance with the invention. It is possible that the basicreason for the superior performance of the aluminum bonded hydridablematerial has to do with the fact that aluminum oxide has a high heat offormation and that oxygen transfer from the aluminum to the activeelement lanthanum of the hydridable material of the composite does notoccur. In contrast metals such as nickel, copper and iron form oxideshaving relatively low heats of formation and in the presence of hydrogenthe oxides of these metals are reduced rather readily with transfer ofoxygen therefrom to the lanthanum or other active metal of thehydridable species in the composite. Such an oxygen transfer isaccompanied by loss of capacity as is illustrated in Table I.

                                      TABLE I                                     __________________________________________________________________________    PELLET PERFORMANCE FOR ALTERNATE BALLASTS AND ALTERNATE PROCESSING                      Pelletizing                                                                         Sinter           Pellet Den.                                                                         Pellet Den.                                                                         Max Poros-                                                                            Poros-                   Ident.                                                                             w/o  Pressure                                                                            Tmp.         Δ                                                                           g/cm.sup.3                                                                          g/cm.sup.3                                                                          density                                                                           ity %                                                                             ity %                    No.  Ballast                                                                            ton/in.sup.2                                                                        °C.                                                                        LaNi.sub.5                                                                             (H/M)                                                                             Prehyd.                                                                             Dehyd.                                                                              g/cm.sup.3                                                                        Prehyd.                                                                           Dehyd.                                                                            Comments             __________________________________________________________________________    LN-3 75 Fe                                                                              10    760 pulverized by                                                                          0.59      3.92  7.29    46.3                                                                              1.6 w/o O                                prehydriding                                              LN-6 75 Cu                                                                              "     "   pulverized by                                                                          0.89      5.51  8.73    36.9                                                                              0.48 w/o O                               prehydriding                                              HR-462                                                                        LN-8 60 Al                                                                               2    430 pulverized by                                                                          0.99                                                                              2.62  2.52  3.69                                                                              29.1                                                                              31.6                                         prehydriding                                              LN-12                                                                              82.5 Ni                                                                            11    760 pulverized by                                                                          0.92                                                                              4.59        8.77                                                                              47.6    0.18 w/o O                               prehydriding                                              LN-13                                                                              "    "     "   -100, + 200 mesh                                                                       0.92                                                                              5.15  4.90  "   41.3                                                                              44.1                                                                              0.12 w/o O           LN-14                                                                              "    38    "   -200 + 100 mesh                                                                        0.89                                                                              6.30  5.97  "   28.1                                                                              31.9                                                                              Chip before                                                                   sinter               LN-16                                                                              "    16    "   "        0.79                                                                              6.00  5.53  "   31.6                                                                              37.0                     LN-17                                                                              "    "     "   -14, +30 mesh                                                                          0.29                                                                              6.00        "   31.6                         LN-18                                                                              "    "     "   -14 mesh 0.66                                                                              5.76  5.25  "   34.3                                                                              40.2                     LN-15                                                                              82.5 Cu                                                                            38    "   -30, +100 mesh                                                                         0.73                                                                              6.99  6.78  8.79                                                                              20.5                                                                              22.9                                                                              Chip before                                                                   sinter               LN-19                                                                              "    25    "   "        0.61                                                                              6.74  6.45  "   23.4                                                                              26.6                     HR-470                                                                        LN-20                                                                              "    "     "   -14, + 30 mesh                                                                         0.20                                                                              6.99  6.66  "   20.5                                                                              24.2                     LN-21                                                                              "    "     "   -14 mesh 0.47                                                                              6.47  6.36  "   26.6                                                                              27.6                     HR-421                                                                             75 Ni      "   pulverived by                                                                          0.99      5.12  8.71    41.2                                         prehydriding                                              __________________________________________________________________________     d = density = 8.20 (LaNi.sub.5) = 8.90 (Ni) = 8.92 (Cu) = 7.03 (Fe) = 2.7     (Al)                                                                           i/d.sub.max =  (w/o A)/d.sub.A  + (w/o B)/d.sub.B 100                        Porosity = 1 -  d/d.sub.max                                              

EXAMPLE II

The kinetics in hydride/dehydride reactions as between LaNi₅ as loosepowder and the same quantity of LaNi₅ produced as powder metallurgypellets with fine nickel powder in the ratio 25% LaNi₅ and 75% nickel byweight were compared. A dead-end reactor made from a 0.75" diameter tubehaving a wall thickness of 0.0675 inches immersed in a 25° C. water bathwas used. A quantity of 8 grams LaNi₅ was used in each test. Times forhalf reactions (ΔH/M=0.5) were used to measure reaction kinetics. Thefollowing results were obtained:

                  TABLE II                                                        ______________________________________                                        Absorption                                                                                        Half-time (minutes)                                       P/P.sub.A  Powder   Ballasted Pellet                                          ______________________________________                                        1.5        2.3      0.75                                                      2.0        1.3      0.25                                                      4.0        0.41     0.045                                                     6.0        0.25     0.019                                                     ______________________________________                                        Desorption                                                                                        Half-time (minutes)                                       P/P.sub.D  Powder   Ballasted Pellet                                          ______________________________________                                        0.5        2.4      1.3                                                       0.2        1.3      0.70                                                      0.07       0.92     0.5                                                       ______________________________________                                         P.sub.A = measured absorption plateau pressure at 25° C.               P.sub.D = measured desorption plateau pressure at 25° C.          

Two factors are believed to account for the improvement in kineticswhich is made evident by the markedly reduced times shown in Table II:the intimately mixed ballast and the reduced pressure drop in the pelletbed.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

We claim:
 1. A method for extracting hydrogen from a gas streamcontaining hydrogen with or without other gases by a sorption-desorptionprocess using as the hydrogen sorber a bed of a porous, metallurgicallybonded, heat-ballasted hydridable mixture in pellet form consisting ofat least 60% by volume of a heat-ballast metal powder and not more than40% by volume of a hydridable metal.
 2. The method in accordance withclaim 1 wherein said heat ballast material is a metal from the groupconsisting of nickel, copper, iron and aluminum.